Earthing Boss Question

[Buzz1980]

Hi just a question regarding the supplementary earthing in a hazardous area. When you have a earth boss welded to a piece of steel which is the structure of the building to give a bonding connection for equipment in that area they usually say that only two earth cables per earth boss. Is there a reason why they only recommend to have two per earth boss ? as I can’t see any issue with having to bits of kit coming off the earth boss plus a supplementary earth for the cable tray for example? Any help will be much appreciated

 

[Deleted member 26818]

1) You’d need a longer bolt to maintain good conductivity.
2) The longer bolt would be more prone to damage.
3) Conductivity would be through various lugs, rather than direct to to the boss or bolt.
4) if one bolt failed, you’d have numerous cables disconnected rather than just one or two.

 

[Top Cat]

Glad I don't do hazardous areas! Also, your boss seems like a prat!
What is an earth boss?

 

[Top Cat]

1) You’d need a longer bolt to maintain good conductivity.
2) The longer bolt would be more prone to damage.
3) Conductivity would be through various lugs, rather than direct to to the boss or bolt.
4) if one bolt failed, you’d have numerous cables disconnected rather than just one or two.

How do people know this sh*t?

 

[Automation ATO]

1, lightning grounding can be completely used in the building foundation of the steel as the grounding body, but must be the steel bar electrical through welding together, and use the vertical steel bar in the building column as the lead wire, has been connected with the roof lightning protection facilities.

2, a lot of requirements to lead out the steel bar is used to do artificial grounding, that is in the use of the building foundation itself to do grounding can not meet the requirements of the resistance, and even if the artificial grounding, the effect is much worse than the building itself to do grounding body.

Notes:

1. Conduct good electrical connection of the foundation reinforcement, at least ensure that the outer ring is connected. It is recommended to use 12MM round steel for the bonding material, and the welding length is about 150MM.

2, lead off the average distance should be less than 25 meters, if the house is not large, four corners of a lead off the line is good.

3. It is recommended to use 2 16MM rebar or 4 14MM rebar for each lead wire. Make sure that the lead wire connects the grounding body to the roof lightning arrester.

 

[Pete999]

I certainly would have earthed my Boss, preferably 6 feet under.

 

[bomjac]

If you have more than 2 earths, consider fitting an earth bar. Good practice would be to have the earth bar connected to two seperate earth bosses. That allows you to disconnect them one at a time for maintenance or testing.

Generally if there are multiple earth connections required (considered a few panels for example local to the boss) you could connect the grounding in a ring to give only 2 connection at the boss.

 

[Buzz1980]

Hi lads thanks for all responses some really good info there . Earth bar is a good idea, I’m just trying think why they have this spec at most places, for instance all or most the cable used in a hazardous area is armoured cable which is earthed at both ends. So the metal instrument is getting earthed by the armouring of the gland. So as the two bits of kit are next to each other they are both getting earthed through the armouring of there cables but they want a supplementary bond aswell onto both of the bits of kit from a earth boss. Now the closest earth boss is 500mm away from both bits of kit so both supplementary bonds down to there but they are wanting the tray that the cables are running on also bonding so closet boss is the one as above which will give it three cables onto one boss, now for me I really can’t see any issue with this but with EX inspections someone might have a different opinion. The earth boss is a cylinder block welded onto frame then M10 bolt tapped in. As above I can’t see any issue with this ?? Cheers in adavance

 

[FatAlan]

How do people know this sh*t?

The wonders of specialisation! Knowing more and more about less and less.

 

[bomjac]

Speak to the inspectors first of all to see what they will accept. Very often the interpretation of the rules varies between persons and projects. You have a few options.

• Loop the 2nd instrument from the first, (ie radial earth).
• Ground the tray from the boss and then use the tray to tap off a local ground to the instrument. Your tray needs to be bonded to ground anyway. Ground jumpers between jointed cable trays in the same run are not strictly required, but very often demanded (except for a fresh air jump where you need to provide continuity). The tray should be bonded at the start and finish of the main run, and you can tap off a single bond from the main run for small local trays.
• Fit a flat copper or brass bar to the existing boss with a couple of extra tapping pints and run 1 or 2 cables to each.

Size wise, it should be at least the same size as the conductors (up to 16mm), a minimum of 2.5mm. Again, mostly 6mm is used just to keep things simple for all cables up to 6mm. These are the minimum specs, but the client may expect over and above.

 

[Buzz1980]

Yeah think that will be a good idea speak with inspectors first. But I can’t get my head around why this would be such an issue I would totally understand if there were 7 earths onto one boss but to have 3 on one boss with one of the kit being IS and the other control so both not exceeding 50volts the fault current going through the bolt if anything did happen would be very little and they are onto the same bit of metal anyway. Seems like it’s just belts and braces method again ?

 

[Bobster]

Yeah think that will be a good idea speak with inspectors first. But I can’t get my head around why this would be such an issue I would totally understand if there were 7 earths onto one boss but to have 3 on one boss with one of the kit being IS and the other control so both not exceeding 50volts the fault current going through the bolt if anything did happen would be very little and they are onto the same bit of metal anyway. Seems like it’s just belts and braces method again ?

It's a hazardous area, the requirements are strict as even though there are plenty of different types of protection Exe Exd etc and zones etc... the regs are blanketed as they can't tailor them to every application.

Plus, if you've ever seen the aftermath of something going wrong, you can understand why.

 

[Shweblet]

Hi Trig,
IEC/EN 60079-14 - Electrical installations, design, selection and erection covers this, also IEC 61892 (if its for an offshore platform):

Generally if something is in direct metallic contact i.e. A stainless steel electrical enclosure that is bolted to the structure of the rig/platform (that is confirmed to be connected to the equipotential bonding system) you do not need to have a separate earthing conductor to that enclosure, the same as if you have close coupled Exd to Exe enclosures (as long as they are in direct metallic contact - by means of threaded line bushing etc, This also applies to cable tray i.e. If you have earthed one end of the tray, and you are using fixed splice connectors, there is no need to individually strap numerous lengths together.

Different sites have different practices. Also you mentioned 'IS', you are generally only allowed one earthing point for intrinsic safety and depending on the system, it may not be allowed to be connected to the same boss as other circuits.

We generally work on 1 connection per earth boss, the reason is - if one becomes disconnected its only one. It also prevents daisy chaining.